Liquid dispensers for jetting relatively low viscosity fluids such as solder flux, conformal coatings, encapsulants, underfill material, and surface mount adhesives, are known in the art and generally operate to dispense small volumes of liquid material to a substrate by rapidly contacting a valve seat with a valve member to create a distinct, high pressure pulse that ejects a small volume of liquid from the dispenser. As used herein, liquid material jetting refers to rapidly ejecting a discrete mass of liquid material at a high velocity from a dispenser. Jetting is contrasted with extrusion wherein liquid material is dispensed as a continuous, elongate filament, generally referred to as a “bead” of adhesive. While drops can be formed by rapidly opening and closing a valve during extrusion of liquid material, or by using air to break up an extruded bead as it is dispensed, these processes are distinctly different from jetting processes wherein the discrete liquid mass is rapidly ejected directly from the dispenser at a high velocity.
FIGS. 1A and 1B illustrate the operation of a conventional jetting dispenser 10. In FIG. 1A, a valve member 12 is rapidly moved through a fluid channel 14 in the direction of a valve seat 16 having an outlet 18. As the valve member 12 approaches the valve seat 16, liquid material 20 within the channel 14 flows around the valve tip 12a. FIG. 1B depicts the jetting dispenser 10 at the instant the valve tip 12a contacts valve seat 16. The momentum of the impact between valve member 12 and valve seat 16 generates a pressure pulse that causes a small volume of liquid 20a to be ejected through the outlet 18. U.S. Pat. No. 5,747,102 to Smith et al. and U.S. Pat. No. 6,253,957 to Messerly et al., both assigned to the assignee of the present application, are directed to jetting dispensers.
Conventional jetting dispensers require precise timing control to ensure that a consistent volume of liquid material is jetted from the dispenser. For example, if the valve timing is too fast, there is insufficient time for liquid material to refill within the dispenser, resulting in a lower than desired volume of liquid dispensed. Likewise, if the timing is too slow, the resulting volume of liquid is higher than desired. It has long been thought that hot melt adhesive could not be adequately dispensed by a jetting dispenser, due to the high viscosity of hot melt adhesives and the differences in general rheology between hot melt adhesives and liquid materials that have been conventionally used in jetting processes. Accordingly, hot melt adhesives have generally been dispensed by dedicated hot melt adhesive dispensing systems that utilize high pressure to supply adhesive to a dispensing module. Typical pressures are in the range of 400 psi to 1000 psi. A valve within the dispensing module is opened and closed to regulate the flow of the highly pressurized hot melt adhesive through an outlet nozzle. There is a need for methods and apparatus for dispensing highly viscous materials, such as hot melt adhesive, in discrete, small volumes and which overcome these and other drawbacks of conventional dispensing systems.